The present invention relates generally to an electronic circuit for close temperature regulation over a wide thermal range by controlling coolant flow to a non-linear heat exchanger through control of a non-linear electromechanical valve. The present invention relates specifically to a control circuit for a non-linear electromechanical valve so that water coolant flow within a secondary coolant loop may be regulated through a non-linear heat exchanger of such secondary coolant loop in order that water coolant within a reservoir also circulated in a primary coolant loop to water cool computer modules collectively dissipating from 0 kilowatts (all off) to 28 kilowatts (all on) may be temperature regulated in the face of such varying thermal load to a uniformity of approximately .+-.1.degree. C. The goal of the circuit of the present invention is not so much that a single unique and invariant temperature should be perpetually controllably fixedly maintained, but rather that a very uniform temperature regulation on a time scale of hours should be maintainable within an extremely non-linear system incurring extreme and essentially instantaneous variations in thermal load and ambient temperature.
The general environment in which the circuit of the present invention operates and a prior art circuit for accomplishing the regulation of coolant flow through control of an electromechanical valve is shown in FIG. 1. The LOGIC MODULES are water cooled logic assemblies within a digital computer collectively dissipating from 0 to 10's of kilowatts. The heat from the logic gates within the logic modules will be transferred to an aluminum cold plate with water channels machined inside it. The cooling water is pumped by PUMP 2 through the LOGIC MODULES into the RESERVOIR, and back into the LOGIC MODULES as a PRIMARY COOLANT LOOP. Water from the RESERVOIR is also pumped through a HEAT EXCHANGER and regulated ELECTROMECHANICAL VALVE by PUMP 1 as a SECONDARY COOLANT LOOP. The HEAT EXCHANGER transfers the heat from the water in the RESERVOIR, such as arises from the LOGIC MODULES, into the BUILDING WATER supply. The circuit of interest, the circuit of the present invention, is that circuit which controls the position of the motorized ELECTROMECHANICAL VALVE in this secondary cooling loop.
The system of cooling illustrated in FIG. 1 allows the LOGIC MODULES to operate warmer or cooler than the ambient air. Operation at warmer temperatures generally produces a lower mean-time-between-failure for electronic assemblies. Operating at too cool a temperature could mean condensation of the moisture from the air, which moisture could rapidly deteriorate the module epoxy glass dielectric. Ideally, then, the LOGIC MODULES should be cooled as near to the dew point of the ambient air as is possible.
Assuming that the dew point can be determined (one circuit for which is taught as part of the present disclosure), a control circuit is necessary to regulate the ELECTROMECHANICAL VALVE controlled coolant flow, and thus the thermal heat exchange, within the SECONDARY COOLANT LOOP in order that coolant within the RESERVOIR as circulates in the PRIMARY COOLANT LOOP will be maintained in temperature to be the same as the dew point temperature, plus a small increment to provide a margin of safety. The very simplest approach is to use an on-off solenoid ELECTROMECHANICAL VALVE, which is on when the RESERVOIR water temperature rises above an upper limit (such as dew point +5.degree. C.) and off when the RESERVOIR water reaches dew point temperature. This control creates an oscillating temperature, such as is shown in FIG. 2. This temperature oscillation is not desirable for two reasons. First, because of the nature of this method, the more closely the temperature is regulated the more rapidly the solenoid ELECTROMECHANICAL VALVE must cycle. For a reasonable solenoid life a several degree hysteresis must be used. Such a resonable hysteresis raises the average peak temperature to 5.degree. C. to 8.degree. C. above the dew point (above, because the minimum point must be above or equal to the dew point temperature). Secondly, because some hysteresis is mandatory in on-off control of a solenoid valve, the slight resulting oscillation in temperature strains mechanical joints, such as the soldered dies upon the LOGIC MODULES, more than is desirable.
A superior prior art approach to control of the ELECTROMECHANICAL VALVE is a proportional linear circuit. The control circuit illustrated in FIG. 1 is such a simplified representation of a proportional linear circuit. A voltage corresponding to the desired temperature is applied as signal TEMP. REF. as a reference input to error amplifier AMP. A1. The signal output of a thermometer or thermistor, submerged in the water of the RESERVOIR to be regulated, is applied (as a voltage) to the error signal input of the amplifier AMP. A1. The difference signal is amplified and used to drive a d.c. MOTOR M attached to the VALVE V (which form the ELECTROMECHANICAL VALVE) in the correct direction. If the temperature of the water within the RESERVOIR is hotter than desired, the ELECTROMECHANICAL VALVE opens, allowing more water to flow through the HEAT EXCHANGER and therefore transferring more heat to the BUILDING WATER supply. If the temperature of the water within the RESERVOIR is too cold, the ELECTROMECHANICAL VALVE closes more, reducing the water flow and reducing the heat transfer within the HEAT EXCHANGER to the BUILDING WATER SUPPLY.
The prior art system shown in FIG. 1 with a water RESERVOIR, pipes, pumps and other elements such as cold plates on the LOGIC MODULES, has a thermal inertia, and temperature cannot be changed instantaneously. When the error amplifier AMP A1 senses a difference from the set point, and responsively changes the power to the MOTOR M of the ELECTROMECHANICAL VALVE, there is a time lag of up to several seconds before the temperature sensor in the RESERVOIR determines that any change in cooling has occurred. If the gain of the system composed of the temperature sensor, the proportional linear regulator circuit and the electromechanical valve is too high, then the ELECTROMECHANICAL VALVE will open (or close) too far for the actual change in heat load, and temperature overshoot will occur. For very high systems gains, instability or oscillation can result. The system gain is the amount that the position of the ELECTROMECHANICAL VALVE changes relative to the change in the thermometer or thermistor temperature sensor.
Several prior art methods of stopping oscillation in a proportional linear regulated system with high gain are available. Such oscillation as occurs yields a result similar to the temperature variation occurring from the usage of a solenoid, or on-off type, control as illustrated in FIG. 2. A first approach lowers the system gain, or closed loop gain, until stability is achieved. This approach usually yields the result that the temperature reached, or the temperature difference from the set point is unacceptably large. Another prior art approach is to use an anticipator to predict the system response and thusly slow the change in position of the ELECTROMECHANICAL VALVE relative to the anticipated response of the thermal system. A good example of this approach is a household thermostat. The bimetalic spring which senses temperature in such a thermostat is much slower to respond to temperature changes than the air around it. If no anticipator were used, wide home air temperature changes, as much as 5.degree. F. to 10.degree. F. would be seen, disrupting comfort. When installed, a heater is adjusted inside the thermostat which adds heat locally to the thermostat in order to cause it to warm up as fast as the ambient air does. Such a heater anticipates the air change based on the known house and furnace characteristics. This implementation of an anticipator does not work for a complex thermal system with many variable heat input rates, for example LOGIC MODULES producing from 0 to 14 kilowatts each. It should be additionally assumed within the system of FIG. 1 that the coolant BUILDING WATER into the HEAT EXCHANGER may vary in temperature from 35.degree. F. to 55.degree. F., and also vary in flow rate (pressure) from 20 to 30 gallons per minute. Additionally, two sets of LOGIC MODULES might be cooled from the same RESERVOIR (thusly up to 28 kilowatts total thermal load).
First, and even second order, differentiator type control circuits as are present in the prior art do not form the basis for the circuit of the present invention. Besides the aforementioned extreme variations in the supply coolant BUILDING WATER, and the wide variation in the thermal load from 0 to 28 kilowatts, it should be realized that the HEAT EXCHANGER and the ELECTROMAGNETIC VALVE are extremely non-linear. Non-linearity in the HEAT EXCHANGER means that thermal heat transfer is not linear with either coolant temperature or coolant flow. Non-linearity in the ELECTROMECHANICAL VALVE means that neither fluid flow in the secondary loop (nor resultant temperature in the RESERVOIR) is linear with the valve position. The mathematical model for these several non-linearities, should such be capable of construction, would require very complex electronics for a differentiator type control circuit which would allow close temperature regulation over a wide range of variable thermal load (0 to 28 kilowatts), and a variable thermal exchange media of BUILDING WATER (35.degree. F. to 55.degree. F. at differing flow rates and pressures).